If 72 boys can build a wall of $280\text{m}$ length in 21 days, how many boys would take 18 days to build a similar type of wall of length $200\text{m}$?

We can use the chain rule formula:
$$\frac{M_1\times D_1}{W_1}=\frac{M_2\times D_2}{W_2}$$

Where:

* $M_1=72$ boys
* $D_1=21$ days
* $W_1=280\text{m}$
* $D_2=18$ days
* $W_2=200\text{m}$
* $M_2=$ number of boys required

Substituting the values into the equation:

$$\frac{72\times 21}{280}=\frac{M_2\times 18}{200}$$

Solving for $M_2$:

$$\frac{72\times 21}{280}=\frac{18\times M_2}{200}$$

$$\frac{1512}{280}=\frac{18\times M_2}{200}$$

$$5.4=\frac{18\times M_2}{200}$$

$$18\times M_2=5.4\times 200$$

$$18\times M_2=1080$$

$$M_2=\frac{1080}{18}=60$$

So, 60 boys are required.